Phased array antennas are widely used in radar systems and are also being introduced into mobile communications networks using pico and nano cells in order to serve densely populated areas with high speed and high quality of service communications links. Due to their reduced size, pico and nano cells face an increased possibility of interference between adjacent cells; beamforming using phased array antennas is being considered in order to control the directivity of the radio frequency signal beam in order to minimize interference. Phased array antennas are characterized by having a radiation lobe that is assembled from many similar radiating antenna elements, such as slots or dipoles, each of which transmit a radio signal having an individually controlled phase and amplitude. Accurately predictable radiation patterns and beam-pointing directions can thereby be achieved. Radar systems and mobile communication network transmitters using phased array antennas can adopt an optical radio frequency generation and control that enables high signal quality, Q, operations to be performed on a very stable RF signal.
Optical generation of RF signals is based on heterodyning a pair of phase locked optical signals which have an optical frequency difference equal to the desired RF signal frequency. The direction of the resulting beam is controlled by appropriately setting the phase of the RF signal transmitted by each element. The phase is controlled by introducing an appropriate time delay between the optical signals before they are heterodyned. In one approach, a pair of time delayed optical signals are generated by subjecting the optical spectrum generated by a mode-locking laser, MLL, to chromatic dispersion by transmitting it through an optical fibre. Due to their different wavelengths, the various spectral components in the MLL spectrum experience different delays. By selecting a suitable pair of optical components in the spectrum a specific delay can be introduced, giving the resulting RF signal a specific phase. In this approach, the pair of spectral components of the MLL spectrum are selected using a wavelength selective device, like a wavelength selective switch, WSS, which is typically big and very expensive. Since a phased array antenna includes hundreds elements, it easy to see that such an approach becomes prohibitive in terms of cost and space. The use of an optical fibre to introduce the time delay by exploiting the chromatic dispersion profile of the fibre faces limitations on the total delta delay which can be applied and the delay resolution. A longer delta delay can be achieved by increasing the chromatic dispersion, whereas a higher resolution is achieved by reducing it. In another approach, reported in L. Zhuang et al, “Single-Chip Ring Resonator-Based 1×8 Optical Beam Forming Network in CMOS-Compatible Waveguide Technology”, IEEE Photonics Technology Letters, vol. 19, August, 2007, pp 1130-1132, time delay is introduced by exploiting micro ring resonator elements instead of optical fibres and WSS devices, to reduce the space occupancy, cost and power consumption. Ring resonators introduce cumulated chromatic dispersion to generate a group delay in the passing through light. However, this solution faces functional limitations due to the filtering response of the ring resonator, centred at the resonance frequency of the micro-rings. To operate with a low optical loss, the operating wavelength needs to be far from the resonance frequency, which unfortunately is in the part of the ring resonator transmission spectrum where the group delay generated by the ring resonator is substantially lower. As a consequence, cascaded ring resonators are used to obtain a higher value of group delay, thus complicating the design and the control of the device.